In the special embodiment with six stator teeth and four rotor teeth, the switched reluctance motor represents the most economical option for implementing a brushless drive. It was possible to reduce the drawback, which the motor had up until now, namely a high reactive power pulsation, to a small measure through special designs of the magnetic circuits of this motor. Novel circuit topologies became possible in the actuation electronics of this motor through the use of power circuit breakers.
These reasons and its extraordinary robustness predestine this drive principle for use in mass-manufactured products, in particular, in motor vehicles.
The physical principle of torque extraction of this motor type is based on the desire of a magnetic circuit permeated by a magnetic flux to minimize the magnetic resistance (or: reluctance) which is active in the circuit. If a rotor with a rotational angle-dependent magnetic resistance is disposed in a magnetic circuit, a rotational force is exerted upon the rotor if its rotation results in a reduction of the magnetic resistance.
FIG. 1 illustrates an exemplary course of the phase winding inductances of a three-phase reluctance motor having three phase windings, which motor has four rotor teeth and six stator teeth according to FIG. 2.
From IEE PROC., vo. 127B, no. 4, July, 1980, p. 25253-265: P. J. LAWRENSON ET AL. "Variable-speed switched reluctance motors", a calculation method for the behavior of a reluctance motor at a variable speed is known. Characteristic curves for the magnetic flux densities, currents and torques as a function of the rotational angle and of the angular velocity are indicated. A realization for circuit engineering is not indicated.
A microcomputer is used to retrieve the data regarding the switch-on course of the stator phases, which data are stored in a memory, and to therefrom match the switch-on angle and the pulse width of the current pulses switched in phases to the operating behaviors of the motor within certain limits. This matching of the pulse position and of the pulse duration makes it possible to obtain a large torque and speed range. At higher speeds, however, optimum matching can no longer take place in this simple manner. Therefore, towards higher speeds, the torque is reduced very quickly and the motor losses increase.
It is the object of the invention to set the switch-off angle for all operating states of the motor for a reluctance motor of the type described at the outset in such a way that a maximum output or torque yield is accomplished at a high efficiency and torque waviness as well as noise development are reduced.
This object is accomplished by a method for controlling a reluctance motor whose stator is provided with windings upon which, as a function of the angular position and the speed of rotation of the rotor, current pulses calculated according to a predetermined algorithm are impressed. Each winding phase excited according to a predetermined switch-on angle is switched off according to a switch-off angle (.gamma..sub.p) whose angular count starts with the inductance rise of the excited phase winding. Two operating ranges must be differentiated, which are identified as a "chop" range and a block voltage range.
For the method, the switch-off angle (.gamma..sub.p) for all operating ranges of the reluctance motor is calculated, from the actual value of the speed of rotation (n.sub.ist), the actual value of the supply voltage (u.sub.k), the minimum and maximum inductances (L.sub.min, L.sub.max) of a motor phase winding, and the value of the stator pole angle (.beta..sub.s). The minimum and maximum inductances (L.sub.min, L.sub.max) of a motor phase winding and the value of the stator pole angle (.beta..sub.s) are stored in a memory. The limit current (i.sub.G) is first determined from the product of the actual value of the supply voltage (u.sub.k) and the value of the stator pole angle (.beta..sub.s) divided by the product executed in a multiplier of the actual value of the speed of rotation (n.sub.ist) and the difference between the maximum and minimum (L.sub.maz, L.sub.min) inductances of a motor phase winding.
Further, for the invention, the limit current is then supplied to a divider (B5), which determines a normalized current (I.sub.o) from the desired value of the current (i.sub.soll) and the limit current (i.sub.G). The normalized current (I.sub.o) is supplied to a nonlinear functional network (B7), which determines the switching-off time (t.sub.off) with the fixedly deposited values for the stator pole angle (.beta..sub.s), the values for the relative pole overlap of stator teeth and rotor teeth (.alpha..sub.c), the rotor angle (.beta..sub.R), and the minimum and maximum inductances (L.sub.min, L.sub.max). The switching-off time (t.sub.off) is evaluated in a subsequent functional network (B8) with the speed of rotation information (R/L). The output of the subsequent functional network (B8) is supplied to a nonlinear time element (B9) and is used for the triggering of the switching-off process at the stator windings.
With the method of the invention, the angular position of the rotor relative to the stator is measured by means of an angle sensor and therewith an initialization pulse is triggered and supplied to a controller which, by means of an arithmetical unit, first determines the switch-off angle (.alpha..sub.p) at which the switching-off process begins from the angular position of the stator, represented by the stator pole angle .beta..sub.s, and then the angle (.alpha..sub.g) at which the switching-off process must be completed, i. e., which determines the moment for the switching-off process of the current-conducting phase winding.
Additionally, for the method, the speed of rotation is controlled via the current intensity in the stator windings.
Moreover, the switching-off process is triggered when approximately 2/3 of the total time has passed after the symmetrical overlap of stator teeth and rotor teeth.
Further, with the invention, after the measurement of the actual speed of rotation (n.sub.ist), the determination of the desired value of the current (i.sub.soll) is determined and subsequently the speed of rotation (n) is controlled to the desired value by means of the phase winding current (i.sub.s) calculated from the desired value of the current (i.sub.soll).
Details of the invention are explained below in greater detail with reference to the drawing.